1. Field of the Invention
The present invention relates to a fluorescent lamp for a backlight, and particularly, to an External Electrode Fluorescent Lamp (EEFL) having an improved discharge starting (or inception) characteristic of a low temperature lamp by controlling a phenomenon that a conventional EEFL is partially darkened at an initial low temperature, resulting from changing a structure of electrodes, and a LCD device having the same.
2. Background of the Invention
Displays becomes more and more important as a medium for transferring visual information in the current information society. In order to become dominant, displays should have low power consumption, have a thin thickness, be light weight and have a high picture quality.
The display may be classified into a radiative type emitting light by itself, such as a Cathode Bay Tube (CRT), an Electro Luminescence (EL), a Light Emitting Diode (LED), a Vacuum Fluorescent Display (VFD), a Field Emission Display (FED), a Plasma Display Panel (PDP), and a non-radiative type that cannot emit light by itself such as a Liquid Crystal Display (LCD) device.
The LCD device serves to display images by using an optical anisotropy of a liquid crystal. Since the LCD device has advantages that visibility is excellent compared with the conventional CRT, and the average power consumption and heating value are lower than those of the CRT when both of them have the same screen size, currently, the LCD has become the focus of the next-generation display device together with the PDP or the FED.
Since the liquid crystal used for the LCD device is not a light emitting material that emits light by itself, but a light receiving material displaying on the screen by modulating the amount of light incident from outside, the LCD device needs an additional light source for irradiating light onto a light crystal display panel, that is, a lamp unit.
Generally, the LCD device serves to display desired images by supplying data signals according to image information to pixels arranged in a matrix shape and adjusting a light transparency of the pixels.
To this end, the LCD device includes a liquid crystal display panel in which a liquid crystal is infused between an array substrate and a color filter so as to output images, a backlight unit that emits light on a front surface of the panel and which is installed on a rear surface of the liquid crystal display panel, and a plurality of case components fixing the liquid crystal display panel and the backlight unit to each other and coupled to them.
A common electrode and a pixel electrode are formed on the liquid crystal display panel in which the array substrate and the color filter substrate are combined with each other and apply an electric field to a liquid crystal layer. If a voltage from the data signal applied to the pixel electrode is controlled under a state that a voltage is applied to the common electrode, the liquid crystal of the liquid crystal layer may be rotated by dielectric anisotropy according to the electric field between the common electrode and the pixel electrode. Accordingly, light is transmitted or blocked for every pixel thus to display a character or image.
The backlight unit serves to implement a planar light having a uniform brightness from a fluorescent lamp used as a light source. The thickness and the power consumption of the LCD device depend on the backlight unit that is as thin as possible and the utilization efficiency of light.
The backlight unit is classified into a direct type where the fluorescent lamp is disposed on the rear surface of the liquid crystal display panel so as to directly transmit light to the front surface of the panel, and an edge type where the fluorescent lamp is disposed at one lateral surface or both lateral surfaces of the liquid crystal display panel so that light can be reflected, diffused and collected though a light guide plate, a reflection sheet and other sheets and be transmitted to the front surface of the panel.
The edge type backlight unit is easily fabricated. On the other hand, the direct type backlight unit is relatively suitable for a large-sized LCD device when considering the uniformity of light.
Currently, the fluorescent lamp used as the light source of the backlight unit is mainly implemented as a Cold Cathode Fluorescent Lamp (CCFL).
The CCFL is easily applied to the edge type backlight unit, but not suitable for the direct type backlight unit.
The reason is as follows. The CCFL is implemented in a manner such that a lamp electrode and a lamp line are soldered to each other and then a connected portion therebetween is wrapped by a silicon rubber. And, when using the direct type backlight unit that is provided with several lamps, a lot of processing time is needed to solder each lamp and protect each lamp with the silicon rubber. Also, as an integral type lamp holder is applied thereto, it is very difficult to protect the respective connected portions.
In the CCFL, an increasing voltage is applied to electrodes at both ends of the fluorescent lamp until a starting (or inception) voltage is achieved in which a current can be conducted. Then the electrodes are stabilized when the voltage is above the starting voltage. In order to continuously emit light, an AC voltage must be applied to the electrodes and maintained.
The aforementioned is limited to one fluorescent lamp. In the direct type backlight unit, the fluorescent lamps should be respectively operated.
Thus, the backlight unit in which a CCFL is used has been mostly fabricated as the edge type. Accordingly, the lamp has been developed to be easily applicable to the direct type backlight unit. And, an External Electrode Fluorescent Lamp (EEFL) has been proposed.
The conventional EEFL will be explained with reference to FIGS. 1 and 2.
FIG. 1 is a schematic view showing a structure of an EEFL in accordance with the related art.
FIG. 2a is a graph showing changes of a lamp current according to a lamp threshold voltage (Vth) at a normal temperature (25° C.) in the EEFL in accordance with the related art, and FIG. 2b is a graph showing changes of a lamp current according to a lamp threshold voltage (Vth) at a low temperature (0° C.) in the EEFL in accordance with the related art.
An EEFL (10) in accordance with the related art, as shown in FIG. 1. Unlike the structure of the CCFL, in which electrodes are protruded inwardly at both sides of a glass tube 11, external electrodes 13 formed of a metallic material are formed at the outside of both ends of the fluorescent lamp 10. And, ions polarized by the external electrodes 13 are concentrated at both ends of the fluorescent lamp 10, and then the ions are synthesized at the time of zero-crossing by a current of a high voltage. Through this process, the fluorescent lamp 10 emits light.
Here, the external electrodes 13 are not disposed in the fluorescent lamp 10, which indicates that capacitors are disposed at both ends of the fluorescent lamp 10 in an equivalent circuit. Accordingly a plurality of fluorescent lamps 10 can be driven in parallel with each other.
Thus, if there is an inverter having a large capacity, the fluorescent lamp 10 can emit light with the simpler structure and the inverter, compared with the CCFL.
Generally, electrical characteristics of a lamp depend on a filled gas pressure, a material of a glass tube, a diameter of the lamp, a lamp length. Here, an electrode, namely, a length of an external electrode, is the most decisive factor for the electrical characteristics of the EEFL.
If the length (L) of the external electrodes 13 becomes shorter, the lamp discharge voltage (hereafter, referred to as “VL”) increases because of a reduction of a capacitance value of the external electrodes 13. However, a discharge path becomes longer. Accordingly the lamp threshold voltage (hereafter, referred to as “Vth”) increases.
In contrast, if the length (L) of the external electrodes 13 becomes longer, the discharge path becomes shorter and thus the Vth drops. However, the VL drops because of an increase in the capacitance value of the external electrodes 13.
In the case of the direct type LCD device, in order to prevent an initial partial darkness, the VL should be higher than the Vth.
That is, the VL means a lamp stabilization voltage based on a constant tube current, i.e., a lamp voltage when three minutes elapses after the lamp is lightened. The Vth means a minimum lamp voltage necessary to stabilize the lamp after insulation damage on the lamp occurs because the voltage is applied in a gradually increased manner.
However, the EEFL in the related art, as shown in FIGS. 2a and 2b, is changed to have the VL lower than the Vth when a lighting environment is changed to a lower temperature from the normal temperature.
Thus, the fluorescent lamp is not perfectly lightened at the low temperature, which causes a screen quality of the LCD device to be deteriorated.